Is it possible, in case of cobra, to have its venom hardened into a stone? Will that stone amplify light?

Dried snake venom is usually a powder. To harden into a stone, a substance has to crystalize or form a large crystal. Some things can do that. For example, rocky candy is a large crystal made of sugar, but not all substances can form large crystals. I am not aware of any venom that has been made into a crystal.

You are talking a biologic protein and enzymes, wouldn't be practical to reduce to solid and than not likely refract light much less amplify it.

Te venom could be evaporated leaving its dissolved components as a solid. As to 'amplifying light' the simple answer is no.